Usually false etymologies are based on the sound of a word, or the erroneous decomposition of a word that wasn't compound to begin with. An example would be where people say 'history' is a compound of 'his' and 'story' (and not, as it actually is, inherited from Greek 'historia').

Sometimes it'll even reshape the word according to its whim: You could get sparrowgrass from asparagus. For people who think 'history' has 'his' in it, nothing prevents them from creating such abominations as 'herstory'.

Learn to listen for dubious word-histories, and check them against your dictionary, which most likely will know better.